5f electron structure

Unlike the lanthanides, the actinides U, Np, Pu, and Am have a tendency to form linear actinyl dioxo cations with formula MeO and/or Me02. All these ions are paramagnetic except UO and they all have a non-spherical distribution of their unpaired electronic spins. Hence their electronic relaxation rates are expected to be very fast and their relaxivities, quite low. However, two ions, namely NpO and PuOl", stand out because of their unusual relaxation properties. This chapter will be essentially devoted to these ions that are both 5/. Some comments will be included later about UOi (5/°) and NpOi (5/ ). One should note here that there is some confusion in the literature about the nomenclature of the actinyl cations. The yl ending of plutonyl is often used indiscriminately for PuO and PuOl and the name neptunyl is applied to both NpO and NpOi. For instance, SciFinder Scholar" makes no difference between yl compounds in different oxidation states. Here, the names neptunyl and plutonyl designate two ions of the same 5f electronic structure but of different electric charge and... 

Strong resonance line close to zero velocity. The fits - shown as solid line in fig. 32 -were obtained with the following model Fluctuations occur between a strong magnetic ground state with (i.e., with a more localized 5f electron structure) and a... 

Aqueous-solution spectra characteristic of the NpOj and PuOf ions, both having the 5f electronic structure, are shown in Fig. 16.16. Some qualitative similarities in band patterns for these iso-f-electronic ions appear to exist, but detailed analysis of the observed structure in terms of a predictive model is tentative. Electron-transfer bands for NpO, PuOj, and AmO apparently lie at such high energies that they have not been reported, but this type of transition in NpO + (20 800 cm ), PuOi + (19 000 cm ), and AmOi + ( 18 000 cm )... 

The results of atomic spectroscopy as well as atomic quantum calculations have made it possible to determine the ground state of the free actinide atoms. These results (see Table 1 ) (that will be reviewed in the next section of this Chapter) confirm the progressive filling of the 5f shell. From the point of view of the electronic structure of the free atom, therefore, question ii. is solved in the sense of actinides being a series in which the unsaturated 5 f shell is progressively filled (only one or two electrons being accomodated in the 6d shell). 

The remaining exceptions concern the lanthanide series, where samarium at room temperature has a particular hexagonal structure and especially the lower actinides uranium, neptunium, and plutonium. Here the departure from simple symmetry is particularly pronounced. Comparing these three elements with other metals having partly filled inner shells (transition elements and lanthanides), U, Pu, Np have the lowest symmetry at room temperature, normal pressure. This particular crystallographic character is the reason why Pearson did not succeed to fit the alpha forms of U, Pu, and Np, as well as gamma-Pu into his comprehensive classification of metallic structures and treated them as idiosyncratic structures . Recent theoretical considerations reveal that the appearance of low symmetries in the actinide series is intimately linked to the behaviour of the 5f electrons. 

Element, number of 5f electrons and allotrope Structure Lattice parameters ... 

Figure 5 gives the variation of the atomic volume in the actinide series, for the room temperature crystal structures as well as for the ccp and bcc high temperature allotropes, which exist for a number of actinides. The graph is based on the lattice parameters of Table 1, which includes also recent results. The marked dip in the curve from Th to Am illustrates the shrinkage of interactinide distance which is linked to the itinerancy of the 5f electrons in this part of the actinide series. 

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